This invention relates to low-expansion ceramics having a small coefficient of thermal expansion, a high melting point, and a high mechanical strength. The invention also relates to a method of producing the low-expansion ceramics.
With the progress of technology in recent years, demand for material having an excellent heat-resistance and an excellent thermal shock-resistance is increasing. The thermal shock-resistance of ceramics depends on characteristics of the materials, such as the coefficient of thermal expansion, the heat conductivity, the mechanical strength, the Young's modulus, and the Poisson's ratio. The thermal shock-resistance is also affected by the size and shape of the goods concerned and the conditions of heating and cooling or the rate of heat propagation. Among those factors affecting the thermal shock-resistance, the contribution of the coefficient of thermal expansion is especially large, and when the rate of heat propagation is high, the thermal shock-resistance is ruled almost solely by the coefficient of thermal expansion, as well known to those skilled in the art. Accordingly, there is a strong demand for development of low-expansion material with excellent resistance against thermal shock.
As ceramics with a comparatively low thermal expansion, which has a coefficient of thermal expansion in the order of 5 to 20.times.10.sup.-7 (1/.degree.C.) in a temperature range of 25.degree. C. to 800.degree. C., cordierite (MAS) and lithium-aluminum-silicate (LAS) are known. However, such known ceramics have a low melting point, e.g., the melting point of cordierite is 1,450.degree. C. and that of lithium-aluminum-silicate is 1,423.degree. C. For instance, when the ceramics honeycomb is used as a catalyst substrate for catalytic purifying apparatus of automobiles, even the honeycomb substrate using cordierite with a high melting point have been found vulnerable to plugging due to melting if the temperature of the catalyst bed is increased by 100.degree. C. to 200.degree. C. over that of conventional catalyst beds. The increase of the temperature of the catalyst bed is caused by modification of the mounting position of the catalytic converter from the conventional location of under bed to engine proximity for improving the purifying efficiency of the catalyst and by modification of design involving the mounting of a turbo-charger for improving the fuel economy and enging output, which modifications cause an increase in the exhaust gas temperature as compared with that of conventional apparatus. Accordingly, the development of low-expansion material having an excellent heat-resistance, which also has an excellent thermal shock-resistance equivalent to or better than that of cordierite, has been strongly demanded.
On the other hand, ceramics with low-expansion characteristics generally have different values of the coefficient of thermal expansion for different directions of crystalline axes of the crystals forming the ceramics, which different values tend to cause thermal stress in the ceramics, and as the thermal stress exceeds critical strengths of the constituent crystals and grain boundaries, micro cracks are formed in grains and grain boundaries to reduce the mechanical strength thereof. For instance, in the case of ceramic honeycombs for catalytic substrate of automobile catalytic purifying apparatus, breakage may be caused in the ceramics as the ceramic honeycomb is pushed into a catalytic converter, or cracks and breakages are easily caused in the ceramics during automobile running due to vibration and other mechanical shocks. To overcome such difficulties, there is a strong demand for developing low-expansion materials having a high strength available for catalytic substrate.